Recently, in order to improve emission characteristics etc., controlling opening and/or closing timing for an intake valve of an internal combustion engine according to operation conditions has been developed. One example of a method of operating an opening/closing timing for intake and exhaust valves is described by U.S. Pat. No. 6,626,164 B2. The method described in this reference closes an intake valve later during a cylinder cycle than a bottom dead center when an engine operates in medium-load range. On the other hand, when the engine operates in a full-load range, the method closes an intake valve earlier during a cylinder cycle than in a middle-load range. That is, the cylinder air charge amount is decreased by retarding a closing point of an intake valve as engine load is smaller. As a result, by using this method, a need for controlling an amount of inducted air will decrease, thereby pump loss accompanying pressure drop in an intake passage will decrease, and therefore pressure drop in a cylinder in an intake stroke is suppressed, which results in improved engine operating efficiency.
The method described in this reference also retards an opening timing for an intake valve as an engine speed increases within a range in which there is substantially no induced air blowing back through an inlet of a cylinder in an opposite direction when an engine operates in a full-load region.
The method described in the reference can increase engine output with improving operating efficiency of an internal combustion engine. However, increasing an expansion ratio of an internal combustion engine in order to further improve operating efficiency and output is difficult to achieve due to the following issues. Increasing a geometric compression ratio of an internal combustion engine is needed in order to increase an expansion ratio. When a geometric compression ratio of an internal combustion engine is high, if a cylinder air charge filling ratio is in a high range under a low engine speed condition (a condition where a number of rotations is in low range and gas-flow rate in a cylinder is in low range), an air-fuel mixture in the cylinder is over-heated in a compression stroke resulting in abnormal combustion. In the abnormal combustion, a phenomenon can occur, in which unburned air-fuel mixture is auto-ignited before spark ignition, or an unburned part of air-fuel mixture in the cylinder is over-compressed and auto-ignited after spark ignition due to excessive temperatures before flame propagation.
In view of this issue, one object of this invention is improving operating efficiency and output of an internal combustion engine without occurrence of an abnormal combustion.
One aspect of the present description includes a method of controlling an internal combustion engine, having a cylinder accommodating a piston which reciprocates therein, and defining a combustion chamber, an air intake passage through which air is inducted into said cylinder, and an intake valve capable of shutting off air flowing into said cylinder from said air intake passage. The method may include closing said intake valve later during a cylinder cycle than a timing with which an amount of air inducted into said cylinder from said air intake passage would be maximized, and earlier during the cylinder cycle as a desired amount of air to be inducted into said cylinder increases, while an engine is operating at a given engine speed. The method may further include closing said intake valve earlier during a cylinder cycle as the engine speed increases when said desired amount of air to be inducted into said cylinder is at a maximum.
This method overcomes at least some of the disadvantages of above references.
According to this method, in a high-load and low-speed range, an intake valve is closed later, during a cylinder cycle, than a timing with which an amount of air inducted into said cylinder from said air intake passage is maximized, which results in an intake valve being closed at a timing with which a substantial amount of air is blowing back to an intake passage from a cylinder. This makes it possible to avoid the occurrence of abnormal combustion, by decreasing the amount of air inducted into a cylinder without increasing pump loss due to throttle valve closing, within an engine operating range of high-load and low-speed where an abnormal combustion easily occurs. Further, abnormal combustion can be avoided even when a possibility of abnormal combustion is increased by the increasing of the compression ratio, in turn increasing the expansion ratio, in order to improve engine operating efficiency. As a result, this method can further enhance an engine operating efficiency in an engine operation range of high-load and low-speed with a higher expansion ratio and lower pump loss.
Further, a possibility of abnormal combustion in an operation range with higher engine speed than the above-described low engine speed range is decreased. In view of this fact, this method can include closing an intake valve earlier during a cylinder cycle as an engine speed is greater (e.g., as engine speed increases), which restrains blowing back of air and, in turn, increases cylinder air charge amount and increases engine output due to increased combustion of fuel in response to the increased air amount. As a result, according to this method, an engine operation efficiency and engine output are increased in a broad range of engine operation.
In an example embodiment, this method further comprises closing said intake valve earlier during a cylinder cycle as the engine speed increases when the engine speed is a first predetermined speed or less and said desired amount of air to be inducted into said cylinder is at a maximum. The method may further comprise closing said intake valve later during a cylinder cycle as the engine speed increases when the engine speed is greater than said first predetermined speed and the desired amount of air to be inducted into said cylinder is at a maximum at the given engine speed.
According to this method, in response to a decreased possibility of abnormal combustion occurrence as engine speed increases, an intake valve is closed earlier as engine speed increases when an engine speed is equal to a first predetermined speed or less, which limits blowing back of air and increases cylinder air charge amount.
Meanwhile, when the engine speed is greater than said first predetermined speed, in response to an increasing inertia of inducted air flow combined with an increasing engine speed, and retarding of an intake valve closing timing when a desired amount of air to be inducted into said cylinder is at a maximum, the intake valve is closed later as engine speed increases. This will increase cylinder air charge amount, and in turn engine output is increased by combusting an increased amount of fuel in response to increased air amount. As a result, according to this method, engine operation efficiency and engine output are increased in a broad range of engine operation.
In another example embodiment, this method further comprises a step of closing the intake valve at a substantially fixed timing when the desired amount of air to be inducted into the cylinder is a predefined air amount or less.
And in this case, this method may further comprise a step of increasing a pressure in the air intake passage as the desired amount of air to be inducted into the cylinder increases when the desired amount of air to be inducted into the cylinder is the predefined air amount or less. This can ensure that an air amount in a cylinder is the desired amount.
In another example embodiment, this method further comprises a step of opening the intake valve earlier during a cylinder cycle and closing an exhaust valve of said cylinder later during the cylinder cycle as the engine speed increases when the engine speed is a second predetermined speed or less and the desired amount of air to be inducted into the cylinder is at a maximum at the given engine speed.
In this case, an exhaust valve may close before an intake valve opens during the cylinder cycle if an engine speed is the second predetermined speed or less and when the desired amount of air inducted into said cylinder is maximum at the given engine speed.
By shortening an overlap period where both of an intake valve and an exhaust valve are opened as an engine speed is smaller, or by lengthening a negative overlap period where both of an intake valve and an exhaust valve are closed as an engine speed is smaller, an in-cylinder pressure at a timing of an intake valve opening is increased, which will increase an intake air flow rate and, thus, in-cylinder fluidity, thereby inhibiting abnormal combustion.
Meanwhile, because a possibility of abnormal combustion is decreasing as an engine speed is greater, by lengthening an overlap period as an engine speed is greater, scavenging action is enhanced, which results in higher concentration of oxygen in intake air, thereby increasing output torque.
A second aspect of the present description includes a system comprising: an internal combustion engine having a cylinder accommodating a piston which reciprocates therein, and defining a combustion chamber, an air intake passage through which air is inducted into said cylinder, and an intake valve capable of shutting off air flowing into said cylinder from said air intake passage; an intake valve driving mechanism which is configured to cyclically open and close said intake valve; and a controller configured to control said intake valve driving mechanism to: close said intake valve later during a cylinder cycle than a timing with which an amount of air inducted into said cylinder from said air intake passage is maximized, and earlier during the cylinder cycle as the desired amount of air inducted into said cylinder is greater at a given engine speed; and close said intake valve earlier during a cylinder cycle as the engine speed is greater and the desired amount of air inducted into said cylinder is maximum at the given engine speed.
This system also overcomes at least some of the disadvantages of the references described above.
According to this system, in a high-load and low-speed range, an intake valve is closed later during a cylinder cycle than a timing with which an amount of air inducted into said cylinder from said air intake passage is maximized, which results in an intake valve being closed at a timing with which a substantial amount of air is blowing back to the intake passage from a cylinder. This makes it possible to avoid occurrence of an abnormal combustion, by decreasing air amount inducted into a cylinder without increasing pump loss due to throttle valve closing, within an engine operating range of high-load and low-speed where an abnormal combustion easily occurs. Further, this makes it possible to avoid occurrence of an abnormal combustion even when a possibility of abnormal combustion is increased by the increasing of a combustion ratio, in turn increasing an expansion ratio, in order to improving engine operating efficiency. As a result, this system can further enhance an engine operating efficiency in an engine operation range of high-load and low-speed with a higher expansion ratio and lower pump loss.
Further, a possibility of abnormal combustion in an operation range with higher engine speed than the above-described low engine-speed range is decreased. This system closes an intake valve earlier during a cylinder cycle as an engine speed is greater, which restrains blowing back of air and in turn, increases cylinder air charge amount and increases engine output due to increased fuel combustion in response to increased air amount. As a result, according to this system, an engine operation efficiency and engine output are increased in a broader range of engine operation.
In an example embodiment, an internal combustion engine in this system has a geometric compression ratio of 13:1 or greater.
Typically, in an internal combustion engine with a higher geometric compression ratio, in-cylinder temperature can be easily increased by compressing action and thus, there is a high possibility of abnormal combustion. Therefore, it is very effective to adopt this system for such an internal combustion engine with a higher geometric compression ratio.
In an example, an intake valve driving mechanism comprises a camshaft which is driven by a crankshaft that is coupled to said piston and drives an intake valve, and a phase changing mechanism which changes an angular phase of a camshaft relative to an angular phase of said crankshaft, and wherein a controller varies the closing timing of said intake valve by actuating said phase changing mechanism.
As described above, according to the present description, an engine operation efficiency and engine output are increased in a broader range of engine operation.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.